Perfect matte shadow process

[raistlin]

Hi everyone!

Is there a way to render the background alone and paste on that something with Gi?
I mean, something that project shadows on the ground AND also on other objects that's on the background?
Is there a tuto for that?

When i try to render the subject alone and everything else in "matte shadows" (VrayPropertys) i cannot just place the subject on the background... the shadows suxx... Anf of course i mean vegetal objects also...

Thx for the help!

[dgruwier]

I'm honestly not entirely sure what you're asking, but generally if you want to render the background alone, just disable primary visibility on the hero object you don't want to see. The hero object will cast shadows and reflections and GI and everything in the scene like normal, but not be visible. Then in a separate render pass, enable primary visibility on the hero object, but now make the entire background (ground+other objects) a matte surface with Alpha contribution set to -1 and disable shadows and set reflection amount, refraction amount, gi amount to 0. Now you have the hero object on a black background, and the background with just the shadows and GI contribution of the hero object, and you can combine them in comp.

If you strictly want a separate matte shadow pass, create the hero object pass as mentioned above, but render the background with the hero object entirely disabled, and then create yet another render pass where you disable primary visibility on the hero object, make the background a matte surface, but enable shadows + affect alpha, and set reflection, refraction and GI amount to 1. Add the "Matte shadow" render element (which has color, unlike the regular alpha).
In comp, divide the environment with the matte shadow element, then add the hero object as above.

That should cover all the bases. This is all just from memory and I'm not going to test it right now, so proceed at your own risk.

[raistlin]

Super!
I will try, it seems it's more the 2nd method that i need.

Here is a simple test scene for the background
And of course a simple box as a hero object...
I need the entire background to be render one time
And the hero object isolate, and after i'm using After Fx to comp them.

So... i'll try your method!
Many thanks